If the planet is on an eccentric orbit, the orbit-averaged flux is given by

f = L / (4πa2) * 1/√(1-e2)

where L is luminosity of the star, a is semimajor axis, e is eccentricity, given in SI units - if you are working in units scaled to the Earth/Sun system (i.e. solar luminosity, AU, flux relative to Earth's), you can drop the 4π.

Derivation of the factor involving eccentricity is fairly straightforward but requires somewhat better mathematics rendering than is available here

Last edited by Lazarus on 20th November 2009, 6:44 pm; edited 2 times in total (Reason for editing : oops dropped a factor of 4pi)

This formula works too:I = (rē * sigma * T^4)/dē; where I is the insolation in W/mē, r is the radius of the star in meters, T is the temperature of the star in kelvin and d is the semi-major axis in meters.